Normally, voltage converters, also called direct current/direct current converters, DC/DC converters for short, are used to convert a low voltage to a higher voltage. Often, the output voltage to input voltage ratio can be set by selecting a multiplication factor. This is generally done using a few discrete values. By way of example, voltage converters are used to produce flashes with an LED and in the backlight for a liquid crystal display.
Appliances in which voltage converters are used are often portable and are battery operated. They thus do not have a constant voltage at the input for the device to be powered, such as an LED. The operation of the LED needs to be assured independently of a removable input voltage however. For this reason, voltage converters are frequently operated with circuitry for setting the multiplication factor of the voltage converter.
By way of example, the multiplication factor is set by an external microcontroller with an analog/digital converter. Various electrical variables, such as the input and output voltages of the voltage converter and the currents in an LED or in parallel-connected LEDs, are converted into digital signals for this purpose. The drawback of the microcontroller circuit is the high level of complexity. While the microcontroller is performing other tasks, it cannot regulate the voltage converter.
The datasheet “480 mA white LED 1×/1.5×/2× Charge Pump for Backlighting and Camera Flash”, MAX1576 module, No. 19-3326; Rev 0; 6/04, Maxim Integrated Products, USA describes a module which contains a voltage converter and is used to connect up to eight LEDs. The current in the LEDs is derived individually via eight inputs of this module which are routed to constant current regulators, shown as FET current sources in the circuit diagram. The MAX1576 module takes account of the voltages at the input and output of the voltage converter and a voltage ULED when setting the multiplication factor. If one of the eight inputs used for connecting the LEDs is connected to ground inadvertently or on account of a fault, the MAX1576 module sets the output voltage to approximately 5 V. This increases the power consumption.